This invention relates to new and useful improvements in the control of a pulp washing system to remove the maximum amount of dissolved organic and soluble inorganic material present in a pulp slurry undergoing treatment by a pulp washing system while at the same time minimizing the amount of fresh or other reused process water. More specifically, the invention relates to the use of techniques to develop, implement and use a neural network to dynamically monitor and adjust a pulp washing system to obtain an optimum balance between total solids removed from the pulp slurry entering a pulp washing system and the residual unremoved solids present in the pulp slurry as it leaves the washing system, often referred to as soda loss or carry-over.
FIG. 1 illustrates a typical single pulp washer. A pulp slurry stream 22 enters an inlet repulper 10 where it is admixed with a reused process water flow 9, interchangeably referred to as filtrate, to form an admixture of pulp and contaminated water solution. One or more repulper beaters are located in the repulper box to thoroughly mix the admixture which then flows over a weir 13 into the washer vat 2. The washer drum 1 is covered by a filter media 12, generally a mesh cloth of plastic or metal called the face, rotating in the direction shown by arrow 3 where part of the drum is submerged in a pulp slurry contained in a vat 2. A lower pressure inside the drum 1, due to a barometric leg or vacuum source hence the name vacuum drum, extracts the contaminated water solution from the pulp slurry with the pulp forming a mat 4, interchangeably called a sheet or cake, on the face of the filter media 12 in the sheet forming zone 14. As the sheet 4 emerges from the slurry, it enters a drying zone 15 where additional water solution is removed from the mat. As the drum rotates, the mat passes into the displacement zone 16. A stream of fresh water 6 (shower flow), or reused process water, is sprayed onto the mat by shower 5 and displaces the more contaminated vat liquor from the mat. The mat then passes another drying zone 17 and finally a discharge zone 18 where the mat is removed from the face by a removal device 19 and discharged to pass to another washer or another part of the process that is not shown and is not related to this invention.
In some cases, a washer will operate singly as described (for example a bleach pulp washer or a pulp decker/thickener), however, in many cases a plurality of washers are combined to form a complete washing system as shown in FIG. 2. Referring to this figure, three washers are operating together to form the washing system where the pulp slurry passes from washer to washer and reused filtrate is passed from washer to washer in the opposite direction, called countercurrent washing. A pulp slurry stream 22 is introduced into the repulper and is admixed with dilution stream 23. The balance of the system is made up of washers 1, 1' and 1'' rotating in directions indicated by arrows 3, 3' and 3'' inside vats 2, 2' and 2'' discharging mats 7, 7' and 7''. Water streams 6, 9' and 21' are introduced via showers 5, 5' and 5'' with the final pulp mat being discharged from the system as pulp slurry stream 24. Typically, the only fresh water is in stream 6. The filtrate removed from the mats on washers 8, 8' and 8'' pass into filtrate storage tanks 20, 20' and 20''. The filtrate from storage tanks 20 and 20' become dilution streams 9 and 21 into repulpers 10 and 10', respectively. Side streams 9' and 21' split off the main dilution streams and pass to showers 5 and 5'. The filtrate from storage tank 20'' becomes dilution stream 23 to repulper 10'' with a side stream 23' that passes out of the system to a chemical recovery process that is not shown and is not considered as a part of this invention.
In the single and multiple pulp washing systems described with reference to FIGS. 1 and 2, there are actually two process cycles that must be considered and controlled. Referring to FIG. 2, one process cycle is the actual pulp mass moving through the pulp washing system with a time cycle of typically less than ten minutes from the time the pulp mass enters the first washer, as pulp slurry 22, until it leaves the last washer, as pulp slurry 24. The second process cycle is the reused wash liquor cycle made up of fresh water and other reused process water, streams 6, 9' and 21', interchangeably called filtrates, which have a time cycle in the range of two to four hours from the time fresh water stream 6 is added on the last shower until the filtrate leaves the first storage tank 20'' as dilution stream 23 and wash liquor stream 23' going to the chemical recovery system that is not shown.
Attempts to control wash water flow 6 by measuring the solids content of the wash liquor stream 23', going to the recovery system, by a measurement means 25, either manually done or by continuous sensor means, is difficult at best due to the tremendous lag times (typically 2-4 hours) between the time a change is made and the results are measured. The actual controls that take place must relate to the control of the shower water applied during the short time of the pulp flow cycle represented by the passing of the pulp slurry from entering pulp slurry stream 22 to exiting pulp slurry stream 24.
Ideally, the pulp slurry stream 24 carries the minimum amount of soluble organic and inorganic materials because these must be reacted with chemicals in a later process stage and replaced when the liquor stream 23' is processed by a spent chemical recovery system. The fewer the soluble materials in the washed pulp stream, the less the expense for chemicals used and chemical make-up in the recovery cycle. The wash liquor stream 23' cannot simply be sewered due to its potentially adverse effect upon the environment. By evaporation, the solubles are separated and water is reused. Therefore, the less water in the wash liquor stream the better. The soluble and insoluble materials in the wash liquor stream are combustible and can be used as a source of energy. In an actual pulp washing system, there is always competition between the amount of spent chemicals recovered and the capacity of the recovery process to evaporate the filtrate produced by the washers. Minimizing the chemicals lost with the pulp leaving the system is obviously prudent; however, reducing this to the absolute minimum would require infinite dilution which is impractical. Compromises must be made, often on an hourly or daily basis, such that the capacity of the recovery process is not exceeded while the chemical losses are minimized.
Prior methods of control of pulp washing systems depended on an operator observing the operation and adjusting the control parameters based upon his own knowledge and past experience. Historically, human operators have only been marginally effective at controlling the black liquor solids content of the liquor side stream (see FIG. 2, stream 23') leaving the system going to the recovery system (not shown). This is due to the lag times (usually 2-4 hours) between changes to the shower flow 6 on the last stage shower 5 and the resulting effect in terms of the measured solids content of the wash liquor stream 23' leaving the first stage filtrate tank 20''. A real problem exists due to the fact that normal short-term fluctuations in the liquor solids are confused with expected long-term shifts in liquor solids that are results of past adjustments that have been made. This confusion results in unnecessary adjustments or the omission of a necessary adjustment.
Later, as a better understanding of the process became known, relational control concepts were developed and used. In relational control, a control factor is calculated from values of certain process variables and the values of controlled process variables are adjusted to bring the control factor to or near to a target value. Two of the most prevalent of these relational concepts are Dilution Factor (DF) control and Displacement Ratio (DR) control.
The development of Dilution Factor is credited to Leintz in an article titled "The Dilution Curve--Its Use in the Correlation of Pulp Washing and Evaporation," published by Waters and Bergstrom in 1955. According to this article, the DF relationship is used to predict spent liquor solids concentration from rotary drum washing systems based upon certain known operating conditions derived from analytical tests performed manually during operating trials. These results could then be used for design considerations or for determining present operating efficiency of a system.
The Displacement Ratio concept was introduced by Perkins, Welsh and Mappus in an article entitled "Brown Stock Washing Efficiency, Displacement Ratio Method of Determination." This method introduced to the industry another method of determining washing efficiency.
Regardless of the relational control concept proposed, it is required that various process conditions be monitored on a continuous basis such that automated control systems can respond in a manner that maintains the optimum slurry washing for the given conditions. Modern instrumentation systems have long been available that will measure, with reasonable accuracy, the flow rates, temperatures of materials, liquid levels within vessels, relative position of actuator devices and concentrations of various fluid process streams. Systems for measuring mass flow rates and concentration of solid streams, such as that leaving the pulp washing device, are also available; however, these devices are of questionable reliability and require verification by manual testing which can be performed on an hourly basis at best. The result is that continuous processes must be controlled using calculated parameters based on empirical relationships that may or may not be related to dynamic control components in the control system.
Both of the DF and DR concepts were addressed in Seymore U.S. Pat. No. 4,207,141 as related to the continuous control of washing systems, however, slightly different definitions of DF and DR were given than normally used. These control concepts were extended as described in Seymore U.S. Pat. No. 4,840,704 which relates to the control of washer speed to control the inlet consistency and improve washer mat formation and increase washing efficiency. In these methods, there is a requirement to continuously and instantaneously determine the consistency of the pulp mat leaving the washer, where consistency is defined as the ratio of solid pulp mass contained in the pulp stream to the total mass rate (pulp and water) contained in the stream expressed as a percentage. Consistencies and weighting factors are assigned and relate to the nonavailability of on-line measuring devices that can accurately and repeatedly measure the solids content of the fibrous mat leaving the face of the rotating washer.
In recent years, there has been a resurgence in the use of statistical control concepts to affect control over operating processes. Initially, the statistical control programs were basically manual operations performed on an hourly basis by operators that allow them to determine that statistically significant changes have occurred. Based upon their past experience, they can decide whether some action, if any, is warranted. Presently, these programs are typically aimed at identifying the need for operator involvement and understanding of the concepts needed to address the problem typically encountered when large lag times exist between the controlled parameter and the variable that is being Controlled.
This invention overcomes the problems of the prior art processes, including manual control, continuous control based upon attempts to continuously measure mat consistency and statistical control, by use of a trained neural network to predict the value of certain process variables that cannot be directly controlled. This invention is closely related to that disclosed in Grayson and Rudd U.S. Pat. No. 5,111,531 entitled "Process Control Using Neural Network" and incorporated herein by reference. Neural networks are developed and trained using a plurality of measurements, both manual and automatic, to consistently provide continuous outputs that are both repeatable and representative of process variables that have previously been assumed or arrived at by correlation.
The neural network controller is trained so that when production rates are changed from one level to another, historical experience is used to adjust the flow rates in a manner that obtains optimum operating conditions at various fractions of the time constant for each particular pulp washing system. Consequently, when operators make changes to the pulp stock input to the pulp washing system, they need no longer merely wait for changes to occur in the liquor stream some hours later to respond manually, but they can allow the system to dynamically adjust for the change as in a feed forward manner eliminating the problems associated with the long lag in response time.
The combination and accomplishment of all of the above is due to the neural network controller looking at a plurality of variables, including, but not limited to, process inputs from the operating control system, historical data, manual inputs from test results, and outputs from statistical analysis on washer operation to predict, for example, values for pulp mat consistency, pulp mat density, soda loss, black liquor solids, dilution factor and displacement ratio that can be used in relational control schemes or other control schemes.
The neural network controller provided in a closed loop control system for pulp washing systems according to this invention adjusts the set points of controlled variables to provide a higher level of process optimization for pulp washing systems than has been achievable in the past.